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<p>默认构造函数是不需要实参就能调用的<a href="initializer_list.html" title="cpp/language/constructor">构造函数</a>（以空的或为每个形参提供默认实参的参数列表定义）。拥有公开默认构造函数的类型是<a href="../named_req/DefaultConstructible.html" title="cpp/named req/DefaultConstructible"><span style="font-style:italic">可默认构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(DefaultConstructible)</span> </a>的。
</p>
<h3><span class="mw-headline" id=".E8.AF.AD.E6.B3.95">语法</span></h3>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">类名</span> <code><b>(</b></code> <code><b>)</b></code> <code><b>;</b></code>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">类名</span> <code><b>::</b></code> <span class="t-spar">类名</span> <code><b>(</b></code> <code><b>)</b></code> <span class="t-spar">函数体</span>
</td>
<td> (2)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">类名</span><code><b>()</b></code> <code><b>=</b></code> <code><b>delete</b></code> <code><b>;</b></code>
</td>
<td> (3)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(C++11 起)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">类名</span><code><b>()</b></code> <code><b>=</b></code> <code><b>default</b></code> <code><b>;</b></code>
</td>
<td> (4)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(C++11 起)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">类名</span> <code><b>::</b></code> <span class="t-spar">类名</span> <code><b>(</b></code> <code><b>)</b></code> <code><b>=</b></code> <code><b>default</b></code> <code><b>;</b></code>
</td>
<td> (5)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(C++11 起)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<p>其中 <span class="t-spar">类名</span> 必须指名当前类（或类模板的当前实例化），或在命名空间作用域或友元声明中声明时，必须是有限定的类名。
</p>
<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
<div class="t-li1"><span class="t-li">1)</span> 类定义中的默认构造函数的声明。</div>
<div class="t-li1"><span class="t-li">2)</span> 类定义之外的默认构造函数的定义（该类必须包含一条声明 <span class="t-v">(1)</span>）。有关构造函数的 <span class="t-spar">函数体</span> 的细节，参见<a href="initializer_list.html" title="cpp/language/constructor">构造函数与成员初始化器列表</a></div>
<div class="t-li1"><span class="t-li">3)</span> 弃置的默认构造函数：如果它被<a href="overload_resolution.html" title="cpp/language/overload resolution">重载决议</a>选择，那么程序编译失败。</div>
<div class="t-li1"><span class="t-li">4)</span> 预置的默认构造函数：即便其他构造函数存在，编译器也会定义隐式默认构造函数。</div>
<div class="t-li1"><span class="t-li">5)</span> 类定义之外的预置的默认构造函数（该类必须包含一条声明 <span class="t-v">(1)</span>）。这种构造函数被当做是<i>用户提供的（user-provided）</i>（见下文以及<a href="value_initialization.html" title="cpp/language/value initialization">值初始化</a>）。</div>
<p>默认构造函数会在<a href="default_initialization.html" title="cpp/language/default initialization">默认初始化</a>和<a href="value_initialization.html" title="cpp/language/value initialization">值初始化</a>中被调用。
</p><p>如果没有对类类型（<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">struct</span></span></span>、<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">class</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">union</span></span></span>）提供任何用户声明的构造函数，那么编译器将始终声明一个作为其类的 <code>inline public</code> 成员的默认构造函数。
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<p>当存在用户声明的构造函数时，用户仍可以通过关键词 <code>default</code> 强制编译器自动生成原本隐式声明的默认构造函数。
</p>
</td>
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td></tr>
</table>
<p>隐式声明（或在其首个声明被预置）的默认构造函数具有<span class="t-rev-inl t-until-cxx17"><span><a href="except_spec.html" title="cpp/language/except spec">动态异常说明</a></span> <span><span class="t-mark-rev t-until-cxx17">(C++17 前)</span></span></span><span class="t-rev-inl t-since-cxx17"><span><a href="noexcept_spec.html" title="cpp/language/noexcept spec">异常说明</a></span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>中所描述的异常说明。
</p>
<h3><span class="mw-headline" id=".E9.9A.90.E5.BC.8F.E5.AE.9A.E4.B9.89.E7.9A.84.E9.BB.98.E8.AE.A4.E6.9E.84.E9.80.A0.E5.87.BD.E6.95.B0">隐式定义的默认构造函数</span></h3>
<p>如果隐式声明的默认构造函数未被定义为弃置的，那么当它被 <a href="definition.html#ODR_.E5.BC.8F.E4.BD.BF.E7.94.A8" title="cpp/language/definition">ODR 式使用</a><span class="t-rev-inl t-since-cxx11"><span>或者被<a href="constant_expression.html#.E5.B8.B8.E9.87.8F.E6.B1.82.E5.80.BC.E6.89.80.E9.9C.80.E8.A6.81.E7.9A.84.E5.87.BD.E6.95.B0.E4.B8.8E.E5.8F.98.E9.87.8F" title="cpp/language/constant expression">常量求值所需要</a></span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>时，编译器会定义它（即生成函数体并编译），且它与拥有空函数体和空初始化器列表的用户定义的构造函数有严格相同的效果。即它调用这个类的各基类和各非静态成员的默认构造函数。<span class="t-rev-inl t-since-cxx11"><span>如果它满足对于 <a href="constexpr.html" title="cpp/language/constexpr">constexpr 构造函数</a>的要求，那么生成的构造函数是 <code>constexpr</code> 的。</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<p>当存在用户定义的构造函数时，用户仍可以通过关键词 <code>default</code> 强制编译器自动生成原本隐式声明的默认构造函数。
</p>
</td>
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td></tr>
</table>
<h3><span class="mw-headline" id=".E5.BC.83.E7.BD.AE.E7.9A.84.E9.9A.90.E5.BC.8F.E5.A3.B0.E6.98.8E.E7.9A.84.E9.BB.98.E8.AE.A4.E6.9E.84.E9.80.A0.E5.87.BD.E6.95.B0">弃置的隐式声明的默认构造函数</span></h3>
<p>如果满足下列任一条件，那么类 <code>T</code> 中隐式声明的<span class="t-rev-inl t-since-cxx11"><span>或预置的</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>默认构造函数<span class="t-rev-inl t-until-cxx11"><span>不被定义</span> <span><span class="t-mark-rev t-until-cxx11">(C++11 前)</span></span></span><span class="t-rev-inl t-since-cxx11"><span>被定义为弃置的</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>：
</p>
<ul><li> <code>T</code> 拥有<span class="t-rev-inl t-since-cxx11"><span>无默认初始化器的</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>引用类型的成员。
</li><li> <code>T</code> 拥有<span class="t-rev-inl t-since-cxx11"><span>无默认成员初始化器的</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>非 <a href="default_initialization.html#const_.E5.AF.B9.E8.B1.A1.E7.9A.84.E9.BB.98.E8.AE.A4.E5.88.9D.E5.A7.8B.E5.8C.96" title="cpp/language/default initialization">const 可默认构造</a>的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span> 成员。
</li><li> <code>T</code> 拥有默认构造函数被弃置，或对于此构造函数有歧义或不可访问的成员<span class="t-rev-inl t-since-cxx11"><span>，且该成员无默认成员初始化器</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>。
</li><li> <code>T</code> 拥有默认构造函数被弃置，或对于此构造函数有歧义或不可访问的直接或虚基类。
</li><li> <code>T</code> 拥有析构函数被弃置，或对于此构造函数不可访问的直接或虚基类。
</li></ul>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<ul><li> <code>T</code> 是至少有一个变体成员有非平凡默认构造函数，而没有任何变体成员拥有默认成员初始化器的<a href="union.html" title="cpp/language/union">联合体</a>。
</li><li> <code>T</code> 是拥有变体成员 <code>M</code> 的<a href="union.html#.E8.81.94.E5.90.88.E4.BD.93.E5.BC.8F.E7.9A.84.E7.B1.BB" title="cpp/language/union">非联合类</a>，该成员拥有非平凡默认构造函数，而包含 <code>M</code> 的匿名联合体没有任何变体成员拥有默认成员初始化器。
</li></ul>
</td>
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td></tr>
</table>
<ul><li> <code>T</code> 是所有变体成员都是 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span> 的<a href="union.html" title="cpp/language/union">联合体</a>。
</li></ul>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<p>当不存在用户定义的构造函数，且隐式声明的默认构造函数非平凡时，用户仍可以通过关键词 <code>delete</code> 禁止编译器自动生成隐式定义的默认构造函数。
</p>
</td>
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td></tr>
</table>
<h3><span class="mw-headline" id=".E5.B9.B3.E5.87.A1.E9.BB.98.E8.AE.A4.E6.9E.84.E9.80.A0.E5.87.BD.E6.95.B0">平凡默认构造函数</span></h3>
<p>如果满足下列所有条件，那么类 <code>T</code> 的默认构造函数是平凡的（即没有任何动作）：
</p>
<ul><li> 构造函数并非用户提供的（即为隐式定义或在它的首个声明中预置的）
</li><li> <code>T</code> 没有虚成员函数
</li><li> <code>T</code> 没有虚基类
</li></ul>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<ul><li> <code>T</code> 没有拥有默认初始化器的非静态数据成员。
</li></ul>
</td>
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td></tr>
</table>
<ul><li> 每个 <code>T</code> 的直接基类都拥有平凡默认构造函数
</li><li> 每个类类型（或其数组类型）的非静态成员都拥有平凡默认构造函数
</li></ul>
<p>平凡默认构造函数是没有任何动作的构造函数。所有与 C 语言兼容的数据类型（POD 类型）都是可以平凡默认构造的。
</p>
<h3><span class="mw-headline" id=".E5.90.88.E6.A0.BC.E7.9A.84.E9.BB.98.E8.AE.A4.E6.9E.84.E9.80.A0.E5.87.BD.E6.95.B0">合格的默认构造函数</span></h3>
 <table class="t-rev-begin">
<tr class="t-rev t-until-cxx11"><td>
<p>被用户声明或者同时被隐式声明且可定义的默认构造函数是合格的。
</p>
</td>
<td><span class="t-mark-rev t-until-cxx11">(C++11 前)</span></td></tr>
<tr class="t-rev t-since-cxx11 t-until-cxx20"><td>
<p>没有被弃置的默认构造函数是合格的。
</p>
</td>
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span><br><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td></tr>
<tr class="t-rev t-since-cxx20"><td>
<p>满足下列所有条件的默认构造函数是合格的：
</p>
<ul><li> 它没有被弃置，且
</li><li> 满足它的所有<a href="constraints.html" title="cpp/language/constraints">关联约束</a>（如果存在），且
</li><li> 没有比它<a href="constraints.html#.E7.BA.A6.E6.9D.9F.E7.9A.84.E9.83.A8.E5.88.86.E6.8E.92.E5.BA.8F" title="cpp/language/constraints">更受约束</a>的默认构造函数。
</li></ul>
</td>
<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td></tr>
</table>
<p>合格的默认构造函数的平凡性确定该类是否为<a href="lifetime.html#.E9.9A.90.E5.BC.8F.E7.94.9F.E5.AD.98.E6.9C.9F.E7.B1.BB.E5.9E.8B" title="cpp/language/lifetime">隐式生存期类型</a>，以及该类是否为<a href="../named_req/TrivialType.html" title="cpp/named req/TrivialType">平凡类型</a>。
</p>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> A
<span class="br0">{</span>
    <span class="kw4">int</span> x<span class="sy4">;</span>
    A<span class="br0">(</span><span class="kw4">int</span> x <span class="sy1">=</span> <span class="nu0">1</span><span class="br0">)</span><span class="sy4">:</span> x<span class="br0">(</span>x<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// 用户定义默认构造函数</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> B<span class="sy4">:</span> A
<span class="br0">{</span>
    <span class="co1">// 隐式定义 B::B()，调用 A::A()</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> C
<span class="br0">{</span>
    A a<span class="sy4">;</span>
    <span class="co1">// 隐式定义 C::C()，调用 A::A()</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> D<span class="sy4">:</span> A
<span class="br0">{</span>
    D<span class="br0">(</span><span class="kw4">int</span> y<span class="br0">)</span><span class="sy4">:</span> A<span class="br0">(</span>y<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
    <span class="co1">// 不会声明 D::D()，因为已经有其他构造函数</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> E<span class="sy4">:</span> A
<span class="br0">{</span>
    E<span class="br0">(</span><span class="kw4">int</span> y<span class="br0">)</span><span class="sy4">:</span> A<span class="br0">(</span>y<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
    E<span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span> <span class="co1">// 显式预置，调用 A::A()</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> F
<span class="br0">{</span>
    <span class="kw4">int</span><span class="sy3">&amp;</span> ref<span class="sy4">;</span> <span class="co1">// 引用成员</span>
    <span class="kw4">const</span> <span class="kw4">int</span> c<span class="sy4">;</span> <span class="co1">// const 成员</span>
    <span class="co1">// F::F() 被隐式定义为弃置的</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    A a<span class="sy4">;</span>
    B b<span class="sy4">;</span>
    C c<span class="sy4">;</span>
<span class="co1">//  D d; // 编译错误</span>
    E e<span class="sy4">;</span>
<span class="co1">//  F f; // 编译错误</span>
<span class="br0">}</span></pre></div></div>
</div>
<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A"> 缺陷报告 </span></h3>
<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。
</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th> DR
</th>
<th> 应用于
</th>
<th> 出版时的行为
</th>
<th> 正确行为
</th></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://wg21.cmeerw.net/cwg/issue2084">CWG 2084</a>
</td>
<td> C++11
</td>
<td> 默认成员初始化器对预置的联合体默认构造函数是否被弃置没有影响
</td>
<td> 它们阻止预置默认构造函数被定义为弃置的
</td></tr></table>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
<ul><li> <a href="initializer_list.html" title="cpp/language/constructor"> 构造函数</a>
</li><li> <a href="initialization.html" title="cpp/language/initialization"> 初始化</a>
<ul><li> <a href="aggregate_initialization.html" title="cpp/language/aggregate initialization"> 聚合初始化</a>
</li><li> <a href="constant_initialization.html" title="cpp/language/constant initialization"> 常量初始化</a>
</li><li> <a href="copy_initialization.html" title="cpp/language/copy initialization"> 复制初始化</a>
</li><li> <a href="default_initialization.html" title="cpp/language/default initialization"> 默认初始化</a>
</li><li> <a href="direct_initialization.html" title="cpp/language/direct initialization"> 直接初始化</a>
</li><li> <a href="list_initialization.html" title="cpp/language/list initialization"> 列表初始化</a>
</li><li> <a href="reference_initialization.html" title="cpp/language/reference initialization"> 引用初始化</a>
</li><li> <a href="value_initialization.html" title="cpp/language/value initialization"> 值初始化</a>
</li><li> <a href="zero_initialization.html" title="cpp/language/zero initialization"> 零初始化</a>
</li></ul>
</li><li> <a href="new.html" title="cpp/language/new"> new</a>
</li></ul>

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